The present invention relates generally to drill bits and, more specifically, to drill bits which are used in conjunction with a core barrel to obtain core samples in concrete or rock formations.
Known drill bits used in obtaining core samples generally comprise a hollow cylindrical body including a shank portion which is provided with threads on one end for attachment of the bit to a drill string and a crown structure attached to the other end of the shank portion which provides an annular cutting surface for the bit. The crown structure is generally of a slightly larger external diameter than the shank portion so the drill string can be easily rotated while in a bore hole.
The crown structure for the drill bit may be a metal matrix composition such as tungsten carbide having numerous small diamonds embedded therein commonly known as an impregnated crown structure or, alternatively, must be a metal matrix composition having larger diamonds set in the surface of the composition known as a surface set bit.
During use of such a bit in obtaining a core sample from a bore hole, it has been the general practice to force water or other fluid such as drilling mud down the bore hole through the interior of the drill bit, allow the water to pass across the cutting surface and then return to the surface about the exterior of the hollow bit. The flow of water helps to cool the cutting surface of the bit and, more importantly, helps to flush away cuttings at the bottom of the bore hole thereby increasing the speed at which the core sample can be obtained.
To allow for the flow of water past the cutting surface, particularly with drill bits having an impregnated type crown structure, the drill bit is typically provided with one or more waterways in the area of the crown structure. Each waterway generally comprises an outer channel on the exterior surface of the drill bit body which extends to the cutting surface of the crown structure, an inner channel on the interior surface of the body extending to the same radial portion of the cutting surface, and a recessed portion in the crown structure extending radially and connecting the inner and outer channels. With such waterways, water may flow downwardly along the channel in the interior surface of the body, radially across the cutting surface in the recessed portion and then upwardly along the channel in the exterior surface of the body.
For further specifics as to the construction of conventional drill bits for obtaining core samples and their modes of operation, reference is made to the text entitled "Basic Procedures for Soil Sampling and Core Drilling" by W. L. Acker III (Acker Drill Co. Inc. 1974), particularly to pages 185 to 201 thereof.
Several problems are associated with conventional drill bits having the type of waterways as described above. During the drilling of a core sample, the cutting surface of the bit is never completely cleared of drill cuttings. These cuttings tend to concentrate on the cutting surface of the bit and sometimes actually burn into the metal matrix of the crown structure producing a scab-like appearance. Such a concentration of cuttings impedes the drilling action of the bit and, with a bit having an impregnated crown structure, prevents the even erosion and exposure of the diamonds within the metal matrix.